With the possession of advantages such as a fast transmission speed, a large information carrying capacity, and a long transmission distance, a single-mode optical fiber has been massively applied in the construction of optical fiber communication networks. With further development of an optical amplification technology and a wavelength division multiplexing technology, an optical fiber communication system continuously develops forward toward a direction of higher transmission power and a longer transmission distance. As an important transmission medium in the optical fiber communication system, the single-mode optical fiber needs to be further improved in aspects of relevant performance indexes, so as to meet requirements of the optical fiber communication system in actual development. An attenuation coefficient and an effective area of an optical fiber are two important performance indexes of the single-mode optical fiber. The smaller the attenuation coefficient of the optical fiber is, the longer the transmission distance of an optical signal carried thereby is. The larger the effective area of the optical fiber is, the weaker the nonlinear effects of the optical fiber are. A large effective area can effectively suppress the nonlinear effects, such as self-phase modulation, four-wave mixing, and cross-phase modulation, so as to ensure the transmission quality of a high-power optical signal. A reduced attenuation coefficient and an enlarged effective area can effectively increase an optical-signal-to-noise ratio (OSNR) in the optical fiber communication system, so as to further improve the transmission quality and increase the transmission distance of the system.
In an optical fiber material, light scattering caused by non-uniformity forms a scattering loss of an optical fiber. Rayleigh scattering of an optical fiber is one of three scattering mechanisms and is linear scattering (which is irrelevant to the frequency of an optical signal). The Rayleigh scattering is characterized in that the magnitude thereof is in reverse proportion to the biquadrate of the wavelength thereof and the loss caused thereby is related to the type and concentration of a doped material. Generally, the lower the concentration of the doped material is, the smaller the loss caused by the Rayleigh scattering is. A “pure silicon core” optical fiber is an optical fiber with an undoped core layer portion (that is, pure silica quartz glass). Theoretically, the Rayleigh scattering of the pure silicon core optical fiber is very similar to intrinsic Rayleigh scattering of a pure quartz glass material, so that the Rayleigh scattering of the pure silicon core optical fiber also significantly reduces the attenuation coefficient of the optical fiber. Meanwhile, by optimizing parameters, such as a diameter of a core layer and a doping concentration of fluorine (F) in a cladding, the optical fiber is provided with a larger effective area. However, generally, a large effective area significantly increases a bending loss of the optical fiber (which includes a macro-bending loss and a micro-bending loss of the optical fiber), especially in a long-wavelength area. In a cabling process, or an actual laying and using process of the optical fiber, if the anti-bending performance of the optical fiber cannot meet the requirement, the loss of a signal is increased, and the transmission quality of the signal cannot be ensured.
In the U.S. Pat. No. 6,917,740, a pure silicon core single-mode optical fiber having improved material viscosity mismatch and a manufacturing method thereof are described. By doping a core layer with chlorine (Cl) and F, a difference value between glass transition temperatures Tg of the core layer and that of a cladding is reduced to less than 200° C., thereby optimizing the attenuation performance of the optical fiber. This patent neither concerns studies and improvements on the bending performance of the optical fiber, nor concerns the optical transmission performance of the optical fiber.
In the U.S. Pat. No. 6,449,415, disclosed is an optical fiber, a core layer of which is doped with Cl and has a relative refractive index difference being a positive value, and a cladding of which is doped with F and has a relative refractive index difference being a negative value, and the optical fiber has the structure that an inner cladding is a depressed cladding. A Cl-doped material of the core layer can effectively reduce mismatch between a core layer material and a cladding material of the optical fiber and reduce additional stress produced in a wire drawing process. Meanwhile, the structure that the inner cladding is a depressed cladding can improve the bending performance of the optical fiber, yet the structure of the depressed cladding has a limited capability in improving the bending performance, and also influences other optical parameters of the optical fiber, for example, a mode field diameter and a cutoff wavelength of the optical fiber. Besides, in a situation that parameters of an outer cladding are unreasonably designed, the depressed inner cladding structure may cause a leakage problem of an LP01 mode (that is, an attenuation coefficient of the single-mode optical fiber is dramatically increased in a long-wavelength area).
In the U.S. Pat. No. 6,947,650, a pure silicon core optical fiber having an F-doped depressed inner cladding is presented, a ratio D/d of a diameter D of the depressed cladding thereof to a diameter d of a core layer is about 8.5 and is in a range less than 10. A ratio of an operating wavelength λop of the optical fiber to a cutoff wavelength λcut of the optical fiber ranges from 1.0 to 1.2. This patent does not describe the performance of the optical fiber, such as attenuation and bending.
Generally, the bending performance of the optical fiber can be improved by the following methods. A first method is to change an MAC value of the optical fiber (that is, a ratio of a mode field diameter to a cutoff wavelength of the optical fiber). The smaller the MAC value is, the better the anti-bending performance of the optical fiber is. However, as the mode field diameter is reduced, the effective area is also reduced, and meanwhile, the cutoff wavelength of the optical fiber must be less than the operating wavelength, so as to ensure a single-mode operating characteristic. Therefore, the room for improving the bending performance of the optical fiber by changing the MAC value of the optical fiber is limited. In a second method, the bending performance can be improved through a double-cladding structure with an inner cladding being a depressed cladding, but a depressed cladding may cause a phenomenon of “leakage in the LP01 mode” to the optical fiber. In a third method, a depressed cladding similar to a trench is added on an inner cladding of the optical fiber, so that a large mode field diameter is ensured, and meanwhile, the bending performance of the optical fiber is improved. This method is widely applied in a bend-insensitive single-mode optical fiber (that is, a G.657 optical fiber), for example, in Chinese Patent No. CN101598834A, U.S. Pat. No. 7,450,807, and European Patent No. EP1978383. It is not found in any relevant patent or document report that a depressed cladding similar to a trench is adopted in a pure silicon core optical fiber to further improve the bending performance of the optical fiber.
Generally, a dopant changes a relative refractive index difference of quartz glass. Dopants, such as germanium (Ge), Cl, and phosphorus (P), can make the relative refractive index difference of the doped quartz glass a positive value, and are referred to as “positive dopants”. Dopants, such as F and boron (B), can make the relative refractive index difference of the doped quartz glass a negative value, and are referred to as “negative dopants”. If one “positive dopant” and one “negative dopant” are simultaneously used to dope the quartz glass, the relative refractive index difference of the doped quartz glass may be a positive value, a negative value, or 0.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.